From 7a2cc02ba8e7c3721bcb168ffeec9b6619f10e43 Mon Sep 17 00:00:00 2001
From: Alexander Myltsev <a.myltsev@gmail.com>
Date: Tue, 22 Nov 2022 22:40:10 +0300
Subject: [PATCH] Setup TSS.

---
 cpu/gdt.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 cpu/gdt.h |  6 ++++++
 mbr.S     |  2 +-
 3 files changed, 61 insertions(+), 1 deletion(-)

diff --git a/cpu/gdt.c b/cpu/gdt.c
index 4b1b6d2..f5b2572 100644
--- a/cpu/gdt.c
+++ b/cpu/gdt.c
@@ -24,6 +24,10 @@ typedef unsigned uint;
 { ((lim) >> 12) & 0xffff, (uint)(base) & 0xffff,      \
   ((uint)(base) >> 16) & 0xff, type, 1, dpl, 1,       \
   (uint)(lim) >> 28, 0, 0, 1, 1, (uint)(base) >> 24 }
+#define SEG16(type, base, lim, dpl) (struct seg_desc_t)  \
+{ (lim) & 0xffff, (uint)(base) & 0xffff,              \
+  ((uint)(base) >> 16) & 0xff, type, 1, dpl, 1,       \
+  (uint)(lim) >> 16, 0, 0, 1, 0, (uint)(base) >> 24 }
 
 struct seg_desc_t seg_desc[6];
 
@@ -34,16 +38,66 @@ void init_seg_desc() {
     seg_desc[SEG_UDATA] = SEG(STA_W,       USER_BASE, 0xffffffff - USER_BASE, DPL_USER);
 }
 
+typedef uint16_t ushort;
+
+struct taskstate {
+    uint link;         // Old ts selector
+    uint esp0;         // Stack pointers and segment selectors
+    ushort ss0;        //   after an increase in privilege level
+    ushort padding1;
+    uint *esp1;
+    ushort ss1;
+    ushort padding2;
+    uint *esp2;
+    ushort ss2;
+    ushort padding3;
+    void *cr3;         // Page directory base
+    uint *eip;         // Saved state from last task switch
+    uint eflags;
+    uint eax;          // More saved state (registers)
+    uint ecx;
+    uint edx;
+    uint ebx;
+    uint *esp;
+    uint *ebp;
+    uint esi;
+    uint edi;
+    ushort es;         // Even more saved state (segment selectors)
+    ushort padding4;
+    ushort cs;
+    ushort padding5;
+    ushort ss;
+    ushort padding6;
+    ushort ds;
+    ushort padding7;
+    ushort fs;
+    ushort padding8;
+    ushort gs;
+    ushort padding9;
+    ushort ldt;
+    ushort padding10;
+    ushort t;          // Trap on task switch
+    ushort iomb;       // I/O map base address
+} tss;
+
 struct gdt_desc_t {
     uint16_t size;
     void* ptr;
 } __attribute__((packed));
 
 void load_gdt() {
+    seg_desc[SEG_TSS] = SEG16(STS_T32A, &tss, sizeof(tss)-1, 0);
+    seg_desc[SEG_TSS].s = 0;
+    tss.ss0 = SEG_KDATA << 3;
+    tss.esp0 = KERN_STACK_BASE;
+    // setting IOPL=0 in eflags *and* iomb beyond the tss segment limit
+    // forbids I/O instructions (e.g., inb and outb) from user space
+    tss.iomb = (ushort) 0xFFFF;
     init_seg_desc();
 
     struct gdt_desc_t gdt_desc;
     gdt_desc.size = sizeof(seg_desc) - 1;
     gdt_desc.ptr = seg_desc;
     asm("lgdt %0": : "m"(gdt_desc));
+    asm("ltr %0": : "r"((ushort)(SEG_TSS << 3)));
 }
diff --git a/cpu/gdt.h b/cpu/gdt.h
index ca1acba..6f3bce7 100644
--- a/cpu/gdt.h
+++ b/cpu/gdt.h
@@ -4,6 +4,11 @@
 #define STA_W     0x2       // Writeable (non-executable segments)
 #define STA_R     0x2       // Readable (executable segments)
 
+// System segment type bits
+#define STS_T32A    0x9     // Available 32-bit TSS
+#define STS_IG32    0xE     // 32-bit Interrupt Gate
+#define STS_TG32    0xF     // 32-bit Trap Gate
+
 #define DPL_USER 3
 
 #define SEG_KCODE 1
@@ -18,6 +23,7 @@
                 (0xC0 | (((lim) >> 28) & 0xf)), (((base) >> 24) & 0xff)
 
 #define USER_BASE 0x400000 // 4 MB
+#define KERN_STACK_BASE 0x90000
 
 #ifndef __ASSEMBLER__
 void load_gdt();
diff --git a/mbr.S b/mbr.S
index 34fa7c5..fea161f 100644
--- a/mbr.S
+++ b/mbr.S
@@ -116,7 +116,7 @@ init_32bit:
     mov %ax, %fs
     mov %ax, %gs
 
-    mov $0x90000, %ebp      // 6. setup stack
+    mov $KERN_STACK_BASE, %ebp      // 6. setup stack
     mov %ebp, %esp
 
     movzwl entry, %esi